Outline

Objective

After anterior cervical discectomy implantation of a spacer is common practice. The majority of these spacers are trapezoid titanium cages. A known phenomenon is settling of the cage into the vertebral endplates leading to decrease and/or angulation of the cervical spinal segment. We observed in 43% of our cases a mild decrease up to 2 mm and in 12% a pronounced decrease of more than 2mm with or without angulation of the implant. In contrast to the thoracic and lumbar spine there is only limited data concerning the load bearing ability of cervical endplates. The aim of our investigation was to gain these data.

Methods

Bone density of 17 cervical vertebrae was estimated by quantitative computer tomography. After embedding of the vertebrae into PMMA each endplate was slowly compressed after a fixed protocol with increasing loading cycles until failure using a metal indenter resembling the form of a newly developed cervical cage. Endpoint was breakage of the endplate established by failure to resist the increasing loading forces produced by the testing machine.

Results

The mean bone density of the 17 cervical vertebrae was 204 with a standard deviation of 52 mg Ca-HA/ml (range 130 – 281). The endplate failed with a mean loading of 1067 N Â± 415 (range 340 – 1550 N). The maximum load well correlates with the bone density (R2=0.7347). With the 97.79 mm2 load bearing surface of the cage we calculate a mean cervical endplate break strength of 10.47 MPa and a 95% - confidence interval 12.66 – 9.51 MPa. An initial settling produced by resting of the anchoring teeth in the cervical endplates was observed in 7 vertebrae at a load of 113 N (Range 50 – 250 N).

Conclusions

In contrast to thoracic and lumbar spine cervical endplates show lower resistance against axial forces. The data are important to establish testing limits for the development of height adjustable intervertebral spacer implants. A patent pending prototype of such a height adjustable implant consisting of only two parts is shown.